With regard to a thermal power plant, one factor to improve thermal efficiency of the plant is a choice of a fuel used in the plant.
A combined cycle plant is a thermal power plant that combines a gas turbine and a steam turbine. Conventional combined cycle plants use liquefied natural gas (referred to as LNG), which contains very few impurities, as a fuel and achieves over 50% net generating efficiency.
The combined cycle plant can achieve high thermal efficiency, as mentioned above, because of its nature, which is a combination of a gas turbine and a steam turbine. However, because it utilizes a gas turbine, the fuel has to be clean to avoid trouble with turbine components which might be caused by impurities contained in the fuel.
On the other hand, coal is a more readily available fuel for the thermal power plant. It is said that coal is more abundant on earth than LNG However, coal usually contains many impurities and conventionally, is not suitable for a fuel in the gas turbine or the combined cycle plant.
Some pilot plants were made to utilize coal as a fuel for the combined cycle plant by using a technology of coal gasification. Such combined plant operates according to an IGCC (Integrated Gasification Combined Cycle). However, so far, net generation efficiency of the IGCC plant is just above 40%. So, still there is a room for development for the IGCC plant.
A conventional coal thermal power plant, which is a thermal power plant utilizing coal, typically uses pulverized coal as a fuel for combustion in a boiler of the plant. The boiler generates steam, and the steam drives a steam turbine and a generator, in the plant. This type of heat engine operates according to a Rankine cycle.
A schematic diagram of the conventional coal thermal power plant as described above is shown in FIG. 6, where numeric 1 is a boiler, 2 is a steam turbine, and 3 is a feedwater system. As shown in FIG. 6, boiler 1, steam turbine 2, and feedwater system 3 operates according to a Rankine cycle.
Boiler 1 includes a steam generator 4, and reheater 5. Coal 6, which is usually pulverized coal, is introduced in boiler 1 and is combusted to produce combustion gas. Steam is generated in the steam generator 4 and reheater 5 by the heat of the combustion gas.
Steam turbine 2 includes a high pressure turbine 8, an intermediate pressure turbine 9, a low pressure turbine 10, and a generator 11. The rotation shaft of the high pressure turbine 8, intermediate pressure turbine 9, low pressure turbine 10, and generator 11 are coupled to one another. The steam generated in steam generator 4, its flow rate controlled by a steam valve 12, is introduced into high pressure turbine 8. The steam flows inside high pressure turbine 8 and generates work as it expands.
The steam exhausted from high pressure turbine 8 returns to boiler 1 in reheater 5 and is reheated as a reheated steam. The reheated steam is then introduced to intermediate pressure turbine 9 to expand and generate work. The steam from intermediate pressure turbine 9 is introduced to low pressure turbine 10 further expand and generate work. The work generated at turbines 8, 9 and 10 drive generator 11.
Feedwater system 3 includes a condenser 13 and a feedwater pump 14. The steam exhausted from low pressure turbine 10 in steam turbine 2 is introduced into condenser 13. The steam condenses in condenser 13 into water as condensed water. Feedwater pump 14 pumps up the condensed water from condenser 13 as feedwater, which is fed to steam generator 4 in boiler 1.
Some modifications for these conventional steam turbine plants, such as hydrogen utilization, have been attempted because of relatively low thermal efficiency of the conventional coal thermal power plant.
Japanese patent publication (Kokai) No. 2-130204 describes an example of modified coal thermal power plant which utilizes hydrogen. In this example, high temperature steam produced by combustion of hydrogen with oxygen is mixed up with steam generated in a boiler to raise the temperature of the steam introduced to the turbine. In this case, inlet steam temperature is about 1470 degrees Fahrenheit, which is obtained by mixing the high temperature steam resulted form combustion of hydrogen with the steam generated in the boiler, whose temperature is about 1100 degrees Fahrenheit.
In general, the higher temperature steam introduced to the steam turbine, the higher thermal efficiency can be achieved with regard to a steam turbine cycle, which is also referred to as a Rankine cycle. However, the highest inlet steam temperature for existing steam turbines in practical use is about 1,150 degrees Fahrenheit because of the heat resisting property of material used in the boiler and the steam turbine.
So far, the cost to produce hydrogen is too high even in view of the improvement in thermal efficiency. Therefore, this kind of modified conventional steam turbine plant has not been in practical use.